Control circuit and control method for touch panel, and touch-type input device and electronic device using the same

ABSTRACT

There is provided a control circuit configured for a touch panel. The touch panel includes: a first resistive film and a second resistive film installed with a gap between the first resistive film and the second resistive film; and a first terminal and a second terminal that extend from two opposing sides of the first resistive film. The control circuit includes: a driving circuit configured to apply a driving voltage between the first terminal and the second terminal; and a current detection circuit configured to generate a digital current detection value that indicates a current amount obtained by subtracting a predetermined current from a panel current flowing between the first terminal and the second terminal.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority under 35 U.S.C § 119(e) toJapanese Patent Application No. 2017-197785, filed on Oct. 11, 2017, theentire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a resistive film-type touch panel.

BACKGROUND

Recent electronic devices such as smart phones, tablet terminals, laptopcomputers, portable audio devices, digital cameras, and the like have atouch panel (touch sensor) for operating the electronic devices bytouching with a finger.

FIG. 1 is a diagram schematically illustrating a resistive film-typetouch panel. The resistive film-type touch panel (hereinafter, simplyreferred to as a “touch panel”) 100 has a first resistive film 102, asecond resistive film 104, a pair of X electrodes 106, and a pair of Yelectrodes 108. For the sake of description, X and Y axes are taken inthe direction shown in the illustration. The first resistive film 102and the second resistive film 104 are arranged to face each other with agap therebetween. The pair of X electrodes 106 are formed along twoopposing sides extending in a Y direction of the first resistive film102. The pair of Y electrodes 108 are formed along two opposing sidesextending in an X direction of the second resistive film 104. Wirings(terminals) XP and XN extend from the pair of X electrodes 106, andwirings YP and YN extend from the pair of Y electrodes 108.

When a user touches at any point (contact point) P_(T), the firstresistive film 102 and the second resistive film 104 contact with eachother at the point P_(T). Contact resistance is indicated by Rc. Whendetecting the X coordinate, a constant voltage (driving voltage) V_(DRV)is applied between the pair of X electrodes 106, and a potential Vxobtained by dividing the constant voltage V_(DRV) by resistors Rx₁ andRx₂ is generated at the contact point P_(T).Vx=V _(DRV) ×Rx ₁/(Rx ₁ +Rx ₂)

Since Rx₁+Rx₂ is panel impedance (referred to as panel resistance Rx)between the wirings XP and XN and is a value unique to the panel,Vx=V_(DRV)×Rx₁/Rx.

Since the resistance Rx₁ has a value corresponding to the X coordinate,the measured voltage Vx indicates the X coordinate. When both thewirings YP and YN are set to high impedance, the potential Vx of thecontact point is observed through the second resistive film 104 in thepotentials of the wirings YP and YN.

When detecting the Y coordinate, the constant voltage V_(DRV) issimilarly applied between the pair of Y electrodes 108, and thepotential Vy of the first resistive film 102 is measured.Vy=V _(DRV) ×Ry ₁/(Ry ₁ +Ry ₂)

Recently, a touch panel compatible with multi-touch has been required.For example, a multi-touch detection technique in a resistive film-typetouch panel is provided. FIG. 2 is an equivalent circuit diagram of atouch panel when multi-touch occurs. Rc₁ and Rc₂ indicate contactresistances, and Ry₂ indicates impedance between two points P₁ and P₂ ofthe second resistive film 104.

As the distance between the two points P₁ and P₂ of the multi-touch islonger, the combined impedance of the first resistive film 102 and thesecond resistive film 104 decreases. Therefore, coordinates of the twopoints may be detected by detecting the combined impedance Zx. In orderto detect the combined impedance Zx, a current (panel current Ix)flowing through the panel is detected. This also applies to the Ycoordinate.

The panel current Ix is given by the following equation.Ix=V _(DRV) /ZxZx=Rx ₁ +Rx ₃ +Rx ₂//(Rc ₁ +Ry ₂ +Rc ₂)

The above equation indicates combined resistance of parallel resistors.

Rx₁+Rx₂+Rx₃=Rx is established.

FIG. 3A and FIG. 3B are diagrams illustrating a relationship between adistance Δx between two points and a panel current Ix. Here, thisrelational equation is schematically indicated by a straight line, butactually, it is a more complicated curve.

The state of one point touch corresponds to zero of a distance Δxbetween two points. The impedance Zx of the panel when the distance Δxbetween the two points is zero is Zx=Rx₁+Rx₂+Rx₃=Rx, it is equal to thatin a non-touch state, and a panel current Ix₀ at that time is given byIx₀=V_(DRV)/Rx.

When the distance Δx between the two points increases, the panel currentIx increases from Ix₀. That is, the current Ix₀ stably flowsirrespective of the presence or absence of touch, and it is referred toherein as a constant current or a constant component.

Recently, the resistance of the resistive film has been reduced and thepanel resistance Rx has been reduced. FIG. 3A illustrates a case wherethe panel resistance Rx is relatively large, and FIG. 3B illustrates acase where the panel resistance Rx is relatively small. The panelcurrent Ix is normalized so that the maximum value becomes the same foreasy comparison. As can be seen from the comparison of FIG. 3A and FIG.3B, as the panel resistance Rx decreases, the sensitivity of the panelcurrent Ix to the change of the distance Δx between the two pointsdecreases.

From another point of view, when the panel current Ix of FIG. 3A andFIG. 3B is quantized with the same current resolution, in FIG. 3B, theeffective resolution of a fluctuation component depending on thedistance between the two points included in the panel current Ix issignificantly reduced. This problem should not be regarded as a generalperception of those skilled in the art.

SUMMARY

The present disclosure provides some embodiments of a control circuitcapable of detecting a fluctuation component of a current caused by atouch with high accuracy.

According to one embodiment of the present disclosure, there is provideda control circuit configured for a touch panel. In the control circuit,the touch panel includes: a first resistive film and a second resistivefilm installed with a gap between the first resistive film and thesecond resistive film; and a first terminal and a second terminal thatextend from two opposing sides of the first resistive film. The controlcircuit includes: a driving circuit configured to apply a drivingvoltage between the first terminal and the second terminal; and acurrent detection circuit configured to generate a digital currentdetection value that indicates a current amount obtained by subtractinga predetermined current from a panel current flowing between the firstterminal and the second terminal.

According to the present embodiment, it is possible to remove or reducea constant component that does not depend on the distance between thetwo points from the panel current and to use a fluctuation componentdepending on the distance between the two points as a detection target.This makes it possible to increase the effective detection accuracy forthe fluctuation component.

The predetermined current may be set based on the current detectionvalue generated in a non-touch state. In a calibration process, byadjusting the value of the predetermined current so that the currentamount indicated by the current detection value approaches zero, it ispossible to make the constant component that does not depend on thedistance between the two points included in the current detection valueclose to zero.

The current detection circuit may include: a current copy circuitconfigured to generate a sense current corresponding to the panelcurrent; a variable current source configured to generate a correctioncurrent corresponding to the predetermined current; an I/V(current/voltage) conversion circuit configured to convert a differencebetween the sense current and the correction current into a sensevoltage; and an A/D converter configured to convert the sense voltageinto the current detection value.

A gain of the I/V conversion circuit may be adjustable. This makes itpossible to effectively use the dynamic range of the A/D converter.

An input range of the A/D converter may be adjustable. Thus, it ispossible to reduce the remaining constant component.

The driving circuit may have a push-pull output stage, and the currentcopy circuit may be further configured to generate a first copy of acurrent flowing through a high-side transistor of the output stage and asecond copy of a current flowing through a low-side transistor of theoutput stage, and to use a difference between the first copy and thesecond copy as the sense current. In a configuration in which thedriving circuit sources (discharges) the panel current, most of thepanel current flows through the high-side transistor, but an idlecurrent that cannot be ignored may flow through the low-side transistor.With this configuration, it is possible to accurately detect the panelcurrent by removing the influence of the idle current of the outputstage.

The I/V conversion circuit may include a transimpedance amplifierconfigured to receive the driving voltage by a first input and receive adifference between the sense current and the correction current by asecond input.

The control circuit may be integrated on a single semiconductorsubstrate. The term “integrated” may include a case where all thecomponents of a circuit are formed on a semiconductor substrate or acase where the main components of a circuit are integrated, and someresistors, capacitors, or the like may be installed outside thesemiconductor substrate in order to adjust circuit constants. Byintegrating the circuit on one chip, it is possible to reduce thecircuit area and to keep the characteristics of the circuit elementsuniform.

According to another embodiment of the present disclosure, there isprovided a touch-type input device. The touch-type input device mayinclude a touch panel; and the control circuit connected to the touchpanel.

According to further another embodiment of the present disclosure, thereis provided an electronic device. The electronic device may include thetouch-type input device.

Further, arbitrarily combining the foregoing components or substitutingthe expressions of the present disclosure with each other between amethod and an apparatus is also effective as an embodiment of thepresent disclosure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram schematically illustrating a resistive film-typetouch panel.

FIG. 2 is an equivalent circuit diagram of a touch panel whenmulti-touch occurs.

FIG. 3A and FIG. 3B are diagrams illustrating a relationship between adistance Δx between two points and a panel current Ix.

FIG. 4 is a block diagram of an electronic device including a touch-typeinput device according to an embodiment of the present disclosure.

FIG. 5 is a diagram illustrating the principle of detecting an Xcoordinate at the time of single touch.

FIG. 6 is a diagram illustrating the principle of detecting Xcoordinates at the time of multi-touch.

FIG. 7A and FIG. 7B are diagrams illustrating signal processing in acontrol circuit.

FIG. 8 is a circuit diagram of a current detection circuit according toone example of the present disclosure.

FIG. 9 is a circuit diagram illustrating a specific configurationexample of the current detection circuit in FIG. 8.

DETAILED DESCRIPTION

Embodiments of the present disclosure will be now described in detailwith reference to the drawings. Like or equivalent components, members,and processes illustrated in each drawing are given like referencenumerals and a repeated description thereof will be properly omitted.Further, the embodiments are presented by way of example only and arenot intended to limit the present disclosure, and any feature orcombination thereof described in the embodiments may not necessarily beessential to the present disclosure.

In the present disclosure, “a state where a member A is connected to amember B” includes a case where the member A and the member B arephysically directly connected or even a case where the member A and themember B are indirectly connected through any other member that does notaffect an electrical connection state between the members A and B ordoes not impair functions and effects achieved by combinations of themembers A and B.

Similarly, “a state where a member C is installed between a member A anda member B” includes a case where the member A and the member C or themember B and the member C are indirectly connected through any othermember that does not substantially affect an electrical connection statebetween the member A and C or the member B and the member C or does notimpair functions and effects achieved by combinations of the members Aand C or the members B and C, in addition to a case where the member Aand the member C or the member B and the member C are directlyconnected.

FIG. 4 is a block diagram of an electronic device 1 including atouch-type input device 4 according to an embodiment of the presentdisclosure. The electronic device 1 is a smart phone, a tablet terminal,a notebook PC, a portable audio player, a digital camera, a digitalvideo camera, or the like, and includes a display panel 2 having aliquid crystal display (LCD) panel or an organic EL panel. Thetouch-type input device 4 is mounted on the electronic device 1 togetherwith the display panel 2.

The touch-type input device 4 includes a touch panel 100 and a controlcircuit 200. The touch panel 100 is arranged on a surface layer of thedisplay panel 2 and functions as a touch-type input device. Thetouch-type input device 4 determines an X coordinate and a Y coordinateof a point touched by a user with a finger, a pen or the like(hereinafter, referred to as a finger 6). The touch panel 100 is afour-wire (four-terminal) resistive film type, and the configurationthereof is as described with reference to FIG. 1.

The touch panel 100 includes a first resistive film 102, a secondresistive film 104, a first terminal XP, a second terminal XN, a thirdterminal YP, and a fourth terminal YN. The first resistive film 102 andthe second resistive film 104 are arranged so as to overlap each otherwith a gap in a Z axis direction perpendicular to an X axis and a Yaxis. Two sides extending in the Y axis direction of the first resistivefilm 102 are connected to the first terminal XP and the second terminalXN. Two sides extending in the X axis direction of the second resistivefilm 104 are connected to the third terminal YP and the fourth terminalYN.

The touch-type input device 4 supports input of single touch touched bythe user at one point and multi-touch touched by the user at two points(or three or more points). The control circuit 200 is connected to thefirst terminal XP to the fourth terminal YN of the touch-type inputdevice 4 and detects an X coordinate and a Y coordinate of a pointtouched by the user.

The control circuit 200 includes a coordinate detection circuit 202, andis a functional integrated circuit (IC) integrated on one semiconductorsubstrate. The coordinate detection circuit 202 applies an appropriateelectrical signal to the touch panel 100 and detects coordinates (orgesture input) based on the presence or absence of touch and anelectrical change occurring in the touch panel 100 according to touchedcoordinates. The coordinate detection circuit 202 mainly operates in thefollowing four modes.

φ1. Single touch (one point touch), X coordinate detection mode

φ2. Single touch, Y coordinate detection mode

φ3. Multi-touch, X coordinate detection mode

φ4. Multi-touch, Y coordinate detection mode

The coordinate detection circuit 202 includes a driving circuit 204, acurrent detection circuit 206, a voltage detection circuit 208, and asignal processing part 210. A part or the whole of the processing of thesignal processing part 210 may be entrusted to a processor installedoutside the control circuit 200.

The functions of the driving circuit 204, the current detection circuit206, and the voltage detection circuit 208 in each mode will bedescribed.

(Single Touch Detection)

FIG. 5 is a diagram illustrating the principle of detecting an Xcoordinate at the time of single touch. In the mode φ1, the drivingcircuit 204 applies a driving voltage (bias voltage) V_(DRV) between thefirst terminal XP and the second terminal XN. For example, the drivingcircuit 204 may supply a constant voltage V_(REG) corresponding to thedriving voltage V_(DRV) to the first terminal XP, and may ground thesecond terminal XN.

In this state, when the user touches a certain coordinate P_(T) at onepoint, a potential V_(T) of the coordinate P_(T) is given by thefollowing equation (1).V _(T) =V _(REG) ×Rx ₂/(Rx ₁ +Rx ₂)=V _(REG) ×Rx ₂ /Rx  Eq. (1)

Rx=Rx₁+Rx₂ indicates impedance between the first terminal XP and thesecond terminal XN when not touched, i.e., a resistance value of thefirst resistive film 102. Rx₂ is dependent on a distance between XN andP_(T). When the second terminal XN is set to X=0, Rx₂ is proportional tothe X coordinate (a distance X from XN). If the distance between XP andXN is X_(MAX), it is represented by:Rx ₂ =Rx/X _(MAX) ×X  Eq. (2)

Substituting Eq. (2) for Eq. (1) yields Eq. (3).V _(T) =V _(REG) ×X/X _(MAX)  Eq. (3)

That is, the potential V_(T) of the P_(T) when the point P_(T) istouched indicates the X coordinate.

In the mode φ1, the third terminal YP and the fourth terminal YN are setto high impedance. Therefore, no current flows through the secondresistive film 104 and the contact resistance R_(C), the potentialdifference also becomes zero, and the potential V_(T) at the point P_(T)appears at the third terminal YP and the fourth terminal YN. The voltagedetection circuit 208 measures a voltage V_(Y) (=V_(T)) of the thirdterminal YP (or the fourth terminal YN), and generates a digital voltagedetection value S1 indicating the voltage V_(Y). The signal processingpart 210 generates an X coordinate of the point P_(T) based on thevoltage detection value S1 (voltage V_(Y)).

In the mode φ2, a Y coordinate of the point P_(T) is detected byreplacing the first resistive film 102 and the second resistive film 104and performing the same measurement. Specifically, the driving circuit204 applies a driving voltage V_(DRV) between the third terminal YP andthe fourth terminal YN, and the voltage detection circuit 208 measures avoltage V_(X) of the first terminal XP (or the fourth terminal YN) atthat time. The signal processing part 210 generates a Y coordinate ofthe point P_(T) based on the voltage detection value S1 (voltage V_(X)).

(Multi-Touch Detection)

FIG. 6 is a diagram illustrating the principle of detecting Xcoordinates at the time of multi-touch. In the mode φ3, similar to themode φ1, the driving circuit 204 applies a driving voltage V_(DRV)between the first terminal XP and the second terminal XN. Also in themode φ3, the third terminal YP and the fourth terminal YN are set tohigh impedance, and the voltage detection circuit 208 measures a voltageV_(Y) of the third terminal YP (and/or the fourth terminal YN) andgenerates a voltage detection value S1.

In this state, when the user touches certain two points P_(T1) andP_(T2), the voltage V_(Y) has a potential corresponding to the twopoints P_(T1) and P_(T2). For example, it may be configured such thatthe voltage V_(Y) indicates a midpoint between the two points P_(T1) andP_(T2).

The current detection circuit 206 generates a digital current detectionvalue S2 corresponding to a panel current Ix flowing between the firstterminal XP and the second terminal XN. In the present embodiment, thecurrent detection value S2 is not the panel current Ix itself but acurrent amount Ix′(=Ix−I_(ADJ)) obtained by subtracting a predeterminedcurrent I_(ADJ) from the panel current Ix. It should be noted that theprocess of subtracting the current I_(ADJ) is performed not in thedigital domain but in the previous analog domain.

The predetermined current I_(ADJ) is determined by calibration asdescribed hereinbelow. It is preferable to adjust the current I_(ADJ) sothat the current Ix′ after subtracting the current I_(ADJ) issubstantially zero in a single touch state (or a non-touch state) wherethe distance Δx between the two points is zero. In other words, it ispreferable that the current I_(ADJ) substantially coincides with theconstant component Ix₀ described with reference to FIG. 3A and FIG. 3B.

The signal processing part 210 generates X coordinates (i.e., X1 and X2)of the two points P_(T1) and P_(T2) based on the voltage detection valueS1 (voltage V_(Y)) and the current detection value S2 (current Ix′).

In the mode φ4, Y coordinates of the two points P_(T1) and P_(T2) aredetected by replacing the first resistive film 102 and the secondresistive film 104 and performing the same measurement. Specifically,the driving circuit 204 applies a driving voltage V_(DRV) between thethird terminal YP and the fourth terminal YN. The voltage detectioncircuit 208 measures a voltage V_(X) of the first terminal XP (or thefourth terminal YN) at that time, and the current detection circuit 206measures a panel current Iy flowing between the third terminal YP andthe fourth terminal YN. The signal processing part 210 generates Ycoordinates (i.e., Y1 and Y2) of the two points P_(T1) and P_(T2) basedon the voltage detection value S1 (voltage V_(X)) and the currentdetection value S2 (current Iy′).

The configuration of the control circuit 200 has been described above.Next, advantages of the control circuit 200 will be described.

FIG. 7A and FIG. 7B are diagrams illustrating signal processing in thecontrol circuit 200. As illustrated in FIG. 7A, the panel current Ix maybe decomposed into a constant component Ix₀ that does not depend on thedistance Δx between the two points and a fluctuation component ΔIdepending on the distance Δx between the two points. The currentdetection circuit 206 uses a current Ix′ corresponding to thefluctuation component ΔI as a detection target. Specifically, thefluctuation component ΔI is extracted by subtracting the predeterminedcurrent I_(ADJ) corresponding to the constant component Ix₀ from thepanel current Ix.

Then, as illustrated in FIG. 7B, a quantization process is performed onthe fluctuation component ΔI. As a result, even when the constantcomponent Ix₀ is large and the fluctuation component ΔI is relativelysmall, the latter can be detected with high resolution. Accordingly, itis possible to improve the detection accuracy of the distance Δx betweenthe two points.

The present disclosure is recognized by the block diagram and thecircuit diagram of FIG. 4, and encompasses various devices and circuitsderived from the above description and is not limited to a specificconfiguration. Hereinafter, a more specific configuration example andmodifications will be described in order to facilitate and clarifyunderstanding of the essence and circuitry operation of the disclosure,rather than to narrow the scope of the present disclosure.

FIG. 8 is a circuit diagram of the current detection circuit 206according to one example of the present disclosure. The currentdetection circuit 206 includes a current copy circuit 230, a variablecurrent source 232, an I/V conversion circuit 234, and an A/D converter236.

The current copy circuit 230 generates a sense current I_(SNS)corresponding to the panel current Ix. The sense current I_(SNS) is areplica of the panel current Ix. The variable current source 232generates a correction current I_(CMP) corresponding to thepredetermined current I_(ADJ). When the sense current I_(SNS) is 1/Ktimes the panel current Ix, the correction current I_(CMP) is also 1/Ktimes the predetermined current I_(ADJ). The variable current source 232may use a current DAC, but is not limited thereto. The current copycircuit 230 may be configured by a current mirror circuit.

The I/V conversion circuit 234 converts a difference (I_(SNS)−I_(CMP))between the sense current I_(SNS) and the correction current Icy into asense voltage V_(SNS). The A/D converter 236 quantizes the sense voltageV_(SNS) and converts it into a current detection value S2.

The I/V conversion circuit 234 is configured to have a variable gain. Inaddition, the A/D converter 236 is configured so that its input range isvariable. The input range of the A/D converter 236 may vary according toa reference voltage V_(REF).

FIG. 9 is a circuit diagram illustrating a specific configurationexample of the current detection circuit 206 in FIG. 8.

The current copy circuit 230 includes transistors forming a currentmirror circuit together with transistors of an output stage 205 of thedriving circuit 204. The output stage 205 has a push-pull type andincludes a high-side transistor M_(H) and a low-side transistor M_(L).The current copy circuit 230 generates a copy of a current flowingthrough each of the high-side transistor M_(H) and the low-sidetransistor M_(L) (i.e., a mirrored current of a current flowing througheach of the high-side transistor M_(H) and the low-side transistorM_(L)), and uses a difference between them as the sense current I_(SNS).

In a configuration in which the driving circuit 204 sources (discharges)the panel current Ix, most of the panel current Ix flows through thehigh-side transistor M_(H), but there may be a case where the idlecurrent I_(IDLE) that cannot be ignored also flows through the low-sidetransistor M_(L). With this configuration, it is possible to accuratelydetect the panel current Ix by removing the influence of the idlecurrent I_(IDLE) of the output stage 205.

The I/V conversion circuit 234 includes a transimpedance amplifier. Thedriving voltage V_(DRV) is input to one input of the transimpedanceamplifier, and a difference I_(SNS)′ between the sense current I_(SNS)and the correction current I_(CMP) is supplied to the other inputthereof. The output (sense voltage) V_(SNS) of the I/V conversioncircuit 234 may be expressed by the following equation.V _(SNS) =V _(DRV) −R×(I _(SNS) −I _(CMP))

Resistance R of the transimpedance amplifier is variable resistance, andthis resistance value corresponds to a gain g of the I/V conversioncircuit 234.

In order to adjust the input range of the A/D converter 236, a variablevoltage source 238 for generating an upper reference voltage V_(REFH) ofthe A/D converter 236 is installed. The variable voltage source 238 maybe configured by, for example, a D/A converter.

The configuration of the current detection circuit 206 has beendescribed above. Next, the calibration of the current detection circuit206 will be described.

A calibration process is performed before the release of the controlcircuit 200. In the calibration process, the predetermined currentI_(ADJ), namely the correction current I_(CMP), and other parameters areoptimized.

First, in the non-touch state, an initial value S2 _(INIT) of thecurrent detection value S2 is acquired in a state in which thecorrection current I_(CMP) is initialized (for example, zero). Thisinitial value S2 _(INIT) indicates the constant component Ix₀.Therefore, the correction current I_(CMP) may be calculated based on theinitial value S2 _(INIT) so that the constant component Ix₀ cancels out.The correction current I_(CMP) may be determined so that the constantcomponent slightly remains.

Furthermore, if it is difficult to obtain an amount of optimumcorrection current I_(CMP) by one measurement, the optimum correctioncurrent I_(CMP) may be determined while increasing the correctioncurrent I_(CMP).

In addition, due to the IC process variation, the actually generatedcorrection current may contain an error and may not match the idealcurrent amount corresponding to the set value. In this case, the errorof the correction current may be measured in the first correction, andthe setting value of the correction current I_(CMP) may be modified sothat the correction current I_(CMP) may be matched in the secondcorrection in consideration of the error.

When the correction current I_(CMP) is set, the gain of the I/Vconversion circuit 234 is subsequently optimized.

The sense voltage V_(SNS) when the constant component remains in thesense current I_(SNS)′ after correction by the correction current Icymay be expressed by the following equation.V _(SNS) =V _(DRV) −R×(I _(FIX) +I _(VAR))

By adjusting the upper reference voltage V_(REFH) and making the upperlimit of the input range of the A/D converter 236 closer toV_(DRV)−R×I_(FIX), it is possible to remove the influence of theconstant component I_(FIX).

The present disclosure has been described above based on the embodiment.It is to be understood by those skilled in the art that the embodimentis merely illustrative and may be differently modified by anycombination of the components or processes thereof, and themodifications are also within the scope of the present disclosure.Hereinafter, these modifications will be described.

In the embodiment, the corrected panel current Ix′ has been described asbeing calibrated so as not to substantially contain the constantcomponent Ix₀, but the present disclosure is not limited thereto and theconstant component Ix₀ is reduced even a little and the effects of thepresent disclosure can be achieved as long as the ratio of thefluctuation component ΔI increases.

In the embodiment, the I/V conversion circuit 234 is not limited to aconfiguration using a transimpedance amplifier. For example, aconversion resistor whose one end is grounded may be arranged in thepath of the corrected sense current I_(SNS)′ and the sense voltageV_(SNS) may be generated based on the voltage of the other end of theconversion resistor.

In the embodiment, it is configured such that the current is detected onthe high potential side (XP, YP), namely the source side, of the drivingcircuit 204, but the present disclosure is not limited thereto and thecurrent may be detected on the low potential side (XN, YN), namely thesink side.

According to the present disclosure in some embodiments, it is possibleto detect a fluctuation component of current caused by multi-touch withhigh accuracy.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the disclosures. Indeed, the novel methods and apparatusesdescribed herein may be embodied in a variety of other forms;furthermore, various omissions, substitutions and changes in the form ofthe embodiments described herein may be made without departing from thespirit of the disclosures. The accompanying claims and their equivalentsare intended to cover such forms or modifications as would fall withinthe scope and spirit of the disclosures.

What is claimed is:
 1. A control circuit configured for a touch panel including a first resistive film, a second resistive film with a gap formed between the first resistive film and the second resistive film, and a first terminal and a second terminal that extend from two opposing sides of the first resistive film, control circuit comprising: a driving circuit configured to apply a driving voltage between the first terminal and the second terminal; a sense current generator configured to generate a sense current corresponding to a panel current flowing between the first terminal and the second terminal; a subtraction circuit configured to subtract a predetermined current from the sense current to generate a corrected sense current in which a ratio of a fluctuation component to a constant component is increased for higher resolution of the fluctuation component upon quantization; an analog amplification circuit configured to convert the corrected sense current into a current detection value for the quantization; and a quantization circuit configured to quantize the current detection value.
 2. The control circuit of claim 1, wherein the predetermined current is set based on the sense current generated in a non-touch state.
 3. The control circuit of claim 1, wherein the sense current generator includes a current copy circuit configured to generate the sense current corresponding to the panel current, wherein the subtraction circuit includes a variable current source configured to generate a correction current corresponding to the predetermined current, wherein the analog amplification circuit includes an I/V conversion circuit configured to convert the corrected sense current into a sense voltage as the current detection value, and wherein the quantization circuit includes an A/D converter configured to convert the sense voltage into a digital current detection value.
 4. The control circuit of claim 3, wherein a gain of the I/V conversion circuit is adjustable.
 5. The control circuit of claim 3, wherein an input range of the A/D converter is adjustable.
 6. A control circuit, for a touch panel, wherein the touch panel comprises: a first resistive film and a second resistive film installed with a gap between the first resistive film and the second resistive film; and a first terminal and a second terminal that extend from two opposing sides of the first resistive film, wherein the control circuit comprises: a driving circuit configured to apply a driving voltage between the first terminal and the second terminal; a panel current detector configured to detect a panel sense current flowing between the first terminal and the second terminal; and a current detection circuit configured to generate a digital current detection value that indicates a current amount obtained by subtracting a predetermined current from the panel current, wherein the current detection circuit includes: a current copy circuit configured to generate a sense current corresponding to the panel current; a variable current source configured to generate a correction current corresponding to the predetermined current; an I/V conversion circuit configured to convert a difference between the sense current and the correction current into a sense voltage; and an A/D converter configured to convert the sense voltage into the current detection value, wherein the driving circuit has a push-pull output stage, and wherein the current copy circuit is further configured to generate a first copy of a current flowing through a high-side transistor of the output stage and a second copy of a current flowing through a low-side transistor of the output stage, and to use a difference between the first copy and the second copy as the sense current.
 7. The control circuit of claim 3, wherein the I/V conversion circuit includes a transimpedance amplifier configured to receive the driving voltage by a first input and receive a difference between the sense current and the correction current by a second input.
 8. The control circuit of claim 1, wherein the control circuit is integrated on a single semiconductor substrate.
 9. A method of controlling a touch panel including a first resistive film, a second resistive film with a gap formed between the first resistive film and the second resistive film, and a first terminal and a second terminal that extend from two opposing sides of the first resistive film, the method comprising: applying a driving voltage between the first terminal and the second terminal; generating a sense current corresponding to a panel current flowing through the first terminal and the second terminal; subtracting a predetermined current from the sense current to generate a corrected sense current in which a ratio of a fluctuation component to a constant component is increased for higher resolution of the fluctuation component upon quantization; converting the corrected sense current into a current detection value; quantizing the current detection value; and detecting multi-touch based on the quantized current detection value.
 10. The method of claim 9, further comprising determining the predetermined current based on the panel current in a non-touch state in a calibration process prior to detecting the multi-touch.
 11. The method of claim 9, wherein quantizing the current detection value includes converting the current detection value into a digital current detection value by an A/D converter, and wherein the method further comprises optimizing a measurement range of the A/D converter in a calibration process prior to detecting the multi-touch. 